This invention relates to improvements whereby video picture distortion, interference and perturbations are prevented when a video signal which is processed in accordance with the teaching of the aforementioned patent application is received by a video receiver of the type having digital vertical synchronizing circuitry.
Broadly, the apparatus disclosed in the aforementioned patent application processes a conventional video signal, such as an NTSC television signal, such that a video picture may be derived and displayed therefrom by a conventional television receiver without additional decoding, decryption, or further processing, yet a conventional video recorder is prevented from recording and playing back that processed video signal. This copy prevention is achieved by increasing and decreasing the length of respective field or frame intervals above and below their conventional lengths. Although conventional television receivers can "follow" such variable frame lengths, conventional video recorders cannot. For example, a conventional frame in the NTSC standard is formed of 525 horizontal line intervals. In accordance with the aforementioned patent application, the frame length is increased by adding more line intervals thereto and is decreased by providing less than the standard 525 lines. The rate at which the frame length increases and decreases, the maximum and minimum lengths or durations of a frame and the number of frames which remain at the maximum and minimum lengths constitute what is referred to in the aforementioned patent application as a "profile". The profile determines frame lengths and varies from time to time.
Notwithstanding such changes in the video frame lengths as well as changes in the profiles which control those lengths, conventional television receivers nevertheless are capable of detecting the vertical synchronizing signals included in each video field and, thus, produce accurate video pictures from those video signals without undesired picture interference. However, the usual servo control systems included in virtually every video tape recorder (VTR) are unable to "lock" onto the vertical synchronizing signals which occur at increasing and decreasing periods in the processed video signal. Thus, whereas accurate video pictures are reproduced by conventional television receivers, the video signals which are processed with varying profiles, as disclosed in the aforementioned patent application, are not accurately recorded and reproduced by conventional VTR's.
Recently, television receivers having digital vertical synchronizing circuits have been introduced. Such circuits generally are of two different types but both typically lock onto the received synchronizing signals after several frames have been received and both "release" the lock-on mode after several frames have been received with noncoinciding synchronizing signals. One type of digital circuit merely counts the horizontal synchronizing pulses included in a field interval. After a predetermined number of such horizontal synchronizing pulses have been counted, the circuitry simply assumes that the beginning (or end) of a field interval has been reached and a vertical retrace signal is generated to retrace to its initial position the scanning electron beam which is used to produce the video picture. For example, in a standard NTSC video signal, once the beginning of a field interval is determined, the vertical retrace signal is generated each time the horizontal synchronizing pulse count reaches 262.5 (or 525). If, by reason of the processing technique disclosed in the aforementioned patent application, the number of line intervals included in a frame is greater than 525, this type of digital vertical synchronizing circuit will generate a vertical retrace signal before an entire field has been received and displayed. Similarly, if less than 525 line intervals are included in a frame of processed video signals, this digital vertical synchronizing circuit will generate a vertical retrace signal some time after the next field has been received. As a result, picture "jumping" will be observed.
But, this type of digital vertical synchronizing circuit has been designed to account for the possibility that a non-standard frame containing more or less than 525 line intervals is received. Vertical retrace is initiated when a 262.5 horizontal sync pulse count is reached only if a predetermined number of frames are received in succession having frame intervals that contain precisely 525 line intervals. Typically, such digital vertical synchronizing circuitry examines two successive frames to verify that each contains 525 lines. If not, the horizontal sync pulse counting operation is not initiated and, thus, a vertical retrace signal is not generated after 262.5 line intervals have been counted. Instead, the vertical retrace signal is produced when the vertical pulses included in the vertical synchronizing interval are detected.
To avoid vertical perturbations in the video picture which may be produced when this type of digital vertical synchronizing circuit is used, the profile which controls the processing of the video signal, as disclosed in the aforementioned patent application, makes certain that two successive frames do not contain 525 line intervals. Thus, this type of digital vertical synchronizing circuit is unable to "lock" onto the received video signal and the horizontal sync pulse counting operation cannot be carried out. Consequently, this type of digital vertical synchronizing circuit is prevented from generating vertical retrace signals at constant, fixed periods when the field periods of the received video signal are varying.
Another type of digital vertical synchronizing circuit likewise initiates a horizontal sync pulse counting operation, but this is done once a "standard" vertical synchronizing interval is detected. The standard vertical synchronizing interval included in the NTSC signal contains six pre-equalizing pulses in the first three line intervals of a field, followed by six vertical pulses in the next three line intervals, followed by six post-equalizing pulses in the next-following three line intervals. If this "standard" vertical synchronizing interval is sufficiently distorted, the digital vertical synchronizing circuit will be unable to detect it and, thus, the horizontal sync pulse counting operation will be inhibited.
This type of digital vertical synchronizing circuit, which senses a standard vertical synchronizing interval, operates to detect when nine pulses of the twelve vertical and post-equalizing pulses are present. If the vertical and post-equalizing pulses are distorted such that no more than eight of these twelve pulses are present, the "standard" vertical synchronizing interval will not be sensed and vertical retrace signals will not be generated at standard, fixed intervals. Thus, the digital vertical synchronizing circuit will be defeated and a vertical retrace signal will be generated at the end of each variable length field interval and not at fixed periods.
It also has been found that, even when analog vertical synchronizing circuits are used to generate vertical retrace signals, brief vertical perturbations may be introduced into the video picture in response to an increase or decrease in a field interval. That is, when the length of a field interval is changed, as by increasing or decreasing the number of line intervals included therein, a brief vertical shift in the displayed video picture occurs; and this appears as a momentary reduction in interlace accuracy. However, this shift generally is not observable and, moreover, disappears within one or two field intervals thereafter. It is believed that this shift is due to a change in the duty cycle of deflection current flowing through the vertical deflection coils of a typical television receiver. The average DC level of the deflection current establishes the center of the video picture. However, when the duty cycle of the deflection current changes, as will occur when the vertical pulses in the received video signal recur at greater or lesser intervals (due to increasing or decreasing frame lengths), the average DC level of the deflection current will change abruptly but soon thereafter will return to the middle of the picture area. This change results in a corresponding vertical movement of the video picture.